In buildings, energy loss takes place primarily through the building envelope. The building envelope consists of doors, windows, and exterior wall and roofing systems.
Walls typically use metal or wood studs to form a frame that can be either load bearing or infill. Multistory buildings can be made of a cast-in-place concrete or steel frame with the exterior perimeter walls being in-filled frame construction between the concrete or steel frame. Once the in-fill frame is installed, exterior sheathing is attached to the exterior side of the frame. On the inside, drywall is often used for the finished surface. This framing system creates a cavity between the exterior sheathing and the drywall. The wall cavity is then filled with batt insulation to insulate the building and improve energy efficiency. However, there are several drawbacks of this system. Framing members create thermal bridging. Batt insulation may not completely fill the cavity wall and over time it can sag leaving no insulation in some portions of the wall. Moisture condensation inside cavity walls is common and may dampen batt insulation within the wall cavity. When this occurs, the damp batt insulation loses most, if not all, insulating properties. In certain climates, a vapor barrier is required to be installed in the wall assembly. While this can help in certain seasons and climates, the year-round changes in temperature, humidity and pressure differential between the interior and exterior of the building make the use of vapor barriers problematic.
Building HVAC systems create pressure differentials between the interior and the exterior of the building. These pressure differentials cause air to move through the exterior wall system. This action is known as HVAC fan pressure. Along with wind, and atmospheric pressure changes, these factors cause air infiltration or exfiltration.
Wind pressure tends to positively pressurize a building on the façade against which it is blowing. And, as wind goes around a corner of a building it cavitates and speeds up considerably, creating especially strong negative pressures at corners and weaker negative pressure on the rest of the building walls and roof.
Stack pressure (or chimney effect) is caused by a difference in atmospheric pressure at the top and bottom of a building due to the difference in temperature, and, therefore, a difference in the weight of columns of air indoors vs. outdoors, especially in winter. In cold climates, stack effect can cause infiltration of air at the bottom of the building and exfiltration at the top. The reverse occurs in warm climates as a result of air-conditioning.
Fan pressure is caused by HVAC system pressurization, usually positively, which is beneficial in warm climates but can cause incremental enclosure problems to wind and stack pressures in climates requiring heating. Infiltration and exfiltration of air in buildings have serious consequences, when they are uncontrolled; the infiltrating air is untreated, and, therefore, can bring pollutants, allergens, and bacteria into buildings. Another serious consequence of infiltration and exfiltration through the building enclosure is condensation of moisture from the exfiltrating air in northern climates, and from infiltrating hot humid air in southern climates, causing mold growth, decay, and corrosion in the wall cavity. This can cause health problems for the building occupants and building material decay with premature building deterioration. Unlike the moisture transport mechanism of diffusion, air pressure differentials can transport hundreds of times more water vapor through air leaks in a building enclosure over the same period of time. This water vapor can condense within a building in a concentrated manner as the air contacts surfaces within the building that are at a temperature below the air's dew-point.
To improve energy efficiency, and to control air infiltration and exfiltration, building codes have recently required the use of air barriers on the exterior sheathing. Air barriers are required on the exterior sheathing to eliminate air exchange. The important features of an air barrier system are: continuity, structural support, air impermeability, and durability. An air barrier has to be continuous and must be interconnected to seal all other elements such as windows, doors and penetrations. Effective structural support requires that any component of an air barrier system must resist the positive or negative structural loads that are imposed on that component by wind, stack effect, and HVAC fan pressures without rupture, displacement or undue deflection. This load must then be safely transferred to the structure. Materials selected to be part of an air barrier system should be chosen with care to avoid materials that are too air-permeable, such as fiberboard, perlite board, and uncoated concrete block. The air permeance of a material is measured using ASTM E 2178 test protocol and is reported in Liters/second per square meter at 75 Pa pressure (cfm/ft2 at 0.3″ w·g or 1.57 psf). The Canadian and IECC codes and ASHRAE 90.1-2010 consider 0.02 L/s·m2 75 Pa (0.004 cfm/ft2 at 1.57 psf), which happens to be the air permeance of a sheet of ½″ unpainted gypsum wall board, as the maximum allowable air leakage for a material that can be used as part of an air barrier system for an opaque enclosure. In order to achieve an airtight structure, the basic materials selected for the air barrier must be highly air-impermeable. The U.S. Army Corps of Engineers (USACE) and the Naval Facilities Command (NAVFAC) have established 0.25 cfm/ft2 at 1.57 psf (1.25 L/s·m2 at 75 Pa) as the maximum air leakage for an entire building (airflow tested in accordance with the USACE/ABAA Air Leakage Test Protocol, which incorporates ASTM E 779); whereas the U.S. Air Force and the International Green Construction Code (IgCC) specify 0.4 cfm/ft2 at 1.57 psf ((2.0 L/s·m2@75 Pa) divided by the area of the enclosure pressure boundary). Materials selected for an air barrier system must perform their function for the expected life of the structure; otherwise they must be accessible for periodic maintenance.
An air barrier, unlike the vapor retarder (which stops air movement, but does not control diffusion), can be located anywhere in an enclosure assembly. If it is placed on the predominantly warm, humid side (high vapor pressure side) of an enclosure or building, it can control diffusion as well, and should be a low-perm vapor barrier material. In such case, it is called an “air and vapor barrier.” If placed on the predominantly cool, drier side (low vapor pressure side) of an enclosure or building, it should be vapor permeable (5-10 perms or greater).
Air barriers can have different vapor permeability ratings. Various building codes bodies classify them as vapor permeable, vapor barriers (vapor impermeable) and vapor retarders (vapor semi-permeable). Elastomeric vapor permeable air barrier have a vapor permeability rating of at least 1-10 perms. Vapor impermeable air barriers have a vapor permeability rating of less than 0.1 perms. Vapor retardant air barriers have a vapor permeability rating of between 0.1 perms and 1 perm.
The ASHRAE Standard 90.1 classifies the 50 states of the USA in at least 8 distinct climate zones. Building codes require a continuous air barrier membrane over the exterior of a building and continuous foam insulation over the structural framing members in all climate zones. However depending on the climate zone, the air barrier requirement can be any one of the three discussed above. For example in hot climates, such a Zones 2 and 3, an air barrier has to be vapor permeable, while in very cold climate, such as Zone 7, an air barrier has to be vapor impermeable. These various factors make it challenging to product manufacturers, designers and contractors to provide the proper solution for each location.
Walls constructed from materials that are very permeable to air, must be air tightened using an applied elastomeric (flexible) coating, either as a specially formulated coating, or a specially formulated air barrier sheet product, or a fluid-applied spray-on or trowel-on material. It has been found that elastomeric polymer coatings are the most effective type of products that meet all of the above criteria.
Thermal performance of the building envelope influences the energy demand of a building in two ways. It affects annual energy consumption, and, therefore, the operating costs for building heating, cooling, and humidity control. It also influences peak energy requirements, which consequently determine the size of heating, cooling and energy generation equipment and in this way has an impact on investment costs. In addition to energy saving and investment cost reduction, a better insulated building provides other significant advantages, including higher thermal comfort because of warmer interior surface temperatures in winter and lower temperatures in summer. This also results in a lower risk of mold growth on internal surfaces.
As can be seen, an air barrier system and building insulation are essential components of the building envelope so that air pressure relationships within the building can be controlled, building HVAC systems can perform as intended, and the occupants can enjoy healthy indoor air quality and a comfortable environment, while reducing energy consumption. HVAC system size can be reduced because of a reduction in the added capacity to cover infiltration, energy loss and unknown factors, resulting in reduced energy use and demand. Air barrier and building insulation systems in a building envelope can also control concentrated condensation and the associated mold, corrosion, rot, and premature failure; and they also improve and promote durability and sustainability. Current building practices typically use a gypsum board or plywood sheathing over the exterior metal or wood framing. In the past, other types of sheathing made of pressed board, asphalt impregnated fiberboard, cement board, aluminum and polyethylene foil-faced foam board have been used over the exterior framing. However due to code requirements to use an air barrier over the exterior sheathing, only materials compatible with elastomeric coatings are being used as sheathing, such as gypsum board and plywood.
Gypsum sheathing has an advantage in that it is fire-resistant; however gypsum has very low insulating value. Gypsum sheathing with glass matt can only resist relatively low impact levels and fails to meet missile impact test requirements associated with coastal construction. Plywood and wood sheathing can meet missile impact test requirements; however, it also has very low insulating value. Both gypsum sheathing and wood sheathing are compatible with and can be coated by liquid applied elastomeric air barriers that meet building code compliance requirements. After plywood or wood sheathing is installed, the sheathing joints are taped and sealed. The exterior of the board is coated with an elastomeric air barrier membrane. Then, to meet code requirements of providing continuous insulation over the structural members, a layer of insulation board is installed. Plastic foam insulation provides good continuous insulation, but does not have any significant structural properties. Therefore, plastic foam insulation is attached over exterior sheathing. However, when this is done, the elastomeric air barrier membrane is penetrated. This can subject the air barrier to moisture and air infiltration and exfiltration risks. To mitigate this problem, aluminum foil insulating boards can be used over the exterior sheathing, such as Thermax polyisocianurate aluminum foil faced insulation board by Dow Chemical. However, aluminum foil insulating boards have a vapor permeability rating of less than 0.04 Perms. Foil faced rigid board insulation provides a good vapor barrier, but cannot be used in climate zones and applications where the air barrier must be vapor permeable. While plastic foam boards are good insulators they have very poor fire resistance properties. Most plastic foam boards are combustible or melt under fire.
Once the building envelope is air tight, architectural wall claddings are installed on the exterior face of the exterior sheathing with the air barrier membrane and continuous plastic foam insulation on it. Stucco, brick, tile, stone, wood siding, metal panels, cement board and EIFS are popular types of exterior wall claddings. With the exception of EIFS, all of these wall claddings have to be mechanically attached to the structural framing members. The mechanical anchors penetrate the air barrier and the sheathing thereby increasing the risk of air infiltration and exfiltration.
Therefore, the new energy code compliant building envelope is comprised of several different materials and components manufactured by different companies and sold and installed by a number of different contractors. This process is labor intensive, time consuming and expensive. As a result, the cost of building an airtight and energy efficient building envelope has risen sharply over the past several years and will continue to rise.
To meet all of the above challenges in all climate zones and applications and to keep cost down, it would be desirable to provide an exterior sheathing product that has an air barrier membrane built into it. It also would be advantageous if the air barrier membrane properties could be adjusted to achieve any desired vapor permeability value; i.e., from a high vapor permeability rating to a low vapor permeable rating to a vapor impermeability rating. It would be desired for the air barrier sheathing to have insulating properties. It would also be desirable that the exterior insulating sheathing product is structurally sound and can resist the positive or negative structural loads that are imposed on a building by wind, stack effect, and HVAC fan pressures without rupture, displacement or undue deflection. It is desirable that these loads are safely transferred to the associated structure. It would be desirable that the exterior sheathing product has fire resistant properties. It would also be desirable that the exterior sheathing allows a wide variety of wall claddings to be attached to it without penetrating the air barrier. In essence the construction industry will benefit tremendously from a sheathing product that has build into it all of the above properties required by building codes. Such a sheathing product would eliminate the current use of multiple products and reduce labor, time and cost of installation.